An electrical contact or contact is an important spare part between two conductors in a switch or a circuit allowing a current to pass through mutual contact, which bears the functions of connecting, carrying and disconnecting a normal current and a fault current. The quality and service life of the contact directly determines the quality and service life of the entire switch or circuit. The electrical contact or contact is mainly used in a relay, a contactor, an air switch, a current limiting switch, a motor protector, a microswitch, an instrument, a computer keyboard, a hand-held set, a household appliance, an automotive electrical appliance (a window switch, a rear-view mirror switch, a lamp switch, a starter motor and other load switch), a leakage protection switch, or the like. The electric contact or contact may be prepared of multiple materials, which mainly include silver, silver-nickel, silver-copper oxide, silver-cadmium oxide, silver-tin oxide, silver-tin oxide-indium oxide, silver-zinc oxide, red copper, brass, phosphor copper, bronze, tin-copper, beryllium copper, copper-nickel, zinc-cupro-nickel, stainless steel, or the like.
In automotive appliances, household appliances, computer keyboards, hand-held sets and other devices, switch components thereof are usually printed circuit boards (PCB) provided with contacts and provided with combinations of contacts and rubber keypads. A circular contact on the PCB is divided into two non-conducting halves by a straight line or curve (like an S-shaped curve). The contact on the keypad is a circle without splitting. A circuit on the PCB can be switched on by using a circular contact of the same diameter on the keypad to make a face-to-face contact with the circular contact on the PCB. The contact on the keypad is made of conductive rubber or metal. The conductive rubber has a larger contact resistance when being contacted with the contact of the PCB. The conductive rubber contact is not suitable for switching on a PCB circuit having a large current (such as current greater than 50 mA). The metal contact has a smaller contact resistance when being contacted with the contact of the PCB. The metal contact not only can be used to switch on a PCB circuit having a smaller current, but also can be used to switch on a PCB circuit having a larger current. However, the metal contact has the problems of unsatisfactory chemical corrosion resistance, unsatisfactory arc-erosion resistance and high production cost at present, thus limiting the applications thereof.
In the atmosphere, a switching element usually generates an electric spark or electric arc when switching on or switching off a circuit. The subsistence of the electric arc phenomenon of the switch will result in contact oxidation and ablation, and may carbonize organic matters in the air, thus producing carbon deposition, which gradually increases a contact resistance of the switch and even causes a circuit break of the switch.
A melting point of tungsten in all pure metals is the maximum. The melting point of pure metals above 1850° C. are: tungsten (a melting point of 3410° C.), rhenium (a melting point of 3180° C.), osmium (a melting point of 3045° C.), tantalum (a melting point of 2996° C.) Ha), molybdenum (a melting point of 2610° C.), niobium (a melting point of 2468° C.), hafnium (a melting point of 2227° C.), vanadium (a melting point of 1900° C.), chromium (a melting point of 1875° C.) and zirconium (a melting point of 1852° C.). The refractory metal has a very low vapor pressure and a relatively low evaporation rate at a high temperature vacuum. The relatively low vapor pressure and evaporation rate of the refractory metal are one reason for us to select the alloy of the refractory metal as a contact material.
Preparation methods for refractory metal alloy include a powder metallurgic method, a vacuum arc melting method and a vacuum electron bombardment melting method and so on, wherein the powder metallurgic method is the most widely used. This is because the method is simple in technology and can obtain fine grain alloy ingot, so as to facilitate further plastic processing. The main process of the powder metallurgic method is as follows: the refractory metal powder (such as tungsten powder and molybdenum powder) is mechanically mixed in proportion; a blank is prepared by a mechanical press or an isostatic press; and then an alloy billet is sintered in a high-temperature sintering furnace introducing hydrogen. A sintering temperature is increased with the increase of tungsten content, within a range of 2150° C. to 2300° C. Finally, a sintered billet is rolled or forged. The use of these methods for the preparation of alloys of refractory metals requires relatively expensive equipment, and the shape of the refractory metal alloy prepared is also shaped. By using the most widely used powder metallurgic method, it is difficult to obtain a refractory alloy sheet (especially a tungsten alloy sheet) having a small thickness. If the thicker sheet of refractory metal alloy is used directly in the production of metal contacts, not only the cost of the raw materials of the metal contacts will be increased, but also it is difficult to cut or punch due to the high hardness of refractory metal alloy.
A patent document with a patent application number of 201220499100.X discloses a “Three-layer Composite Electric Contact”, wherein the contact is provided with a layer of silver plated on a contact surface of a copper-based contact body, so that the contact has better electrical conductivity, and the production cost is saved than that of completing using silver to produce the contact. Although the electrical conductivity and heat conductivity of the silver are highest among all the metals, the silver has poorer atmosphere corrosion resistance and poorer salt-mist resistance. The silver is easily reacted with sulphuretted hydrogen (H2S) in the atmosphere to generate black silver sulfide. When the silver is used as a contact, although the primary surface resistance is smaller, the service life of the silver in the atmosphere is also limited. Although the cost of silver plating is relatively low, the silver is still one of precious metals. Moreover, in such an electric contact, no rubber layer is provided; therefore, this electric contact is not suitable for performing heat vulcanization adhesion and heat vulcanization shaping with rubber to prepare a rubber keypad containing electric contact. Only a contact containing a rubber layer or a contact entirely made of conductive rubber can possibly perform heat vulcanization adhesion and heat vulcanization shaping with other rubber smoothly so as to prepare the rubber keypad containing contact, without causing quality problems like excessive glue and poor adhesion during heat vulcanization adhesion and heat vulcanization shaping.
A patent document with a patent application number of 200580045811.2 discloses a “Flat Primary Battery with Gold-plated Terminal Contact”, which may be applied to, for example, a digital camera. The battery may have a contact containing a lithium anode and a low resistance. The anode and a cathode may present a spirally-crimped sheet form with a baffle therebetween. External anode and cathode contacts are plated by gold so as to improve the contact resistance. Although the electric contact according to the present invention has small resistance, the performance thereof for resisting sparks produced by voltage is not ideal since a melting temperature of gold is poorer than that of tungsten, molybdenum and other refractory metals. Moreover, the expensive price of gold also limits the application range of the electric contact.
A patent document with a patent application number of 201020143455.6 discloses a “Nickel-plated Tungsten Contact”, which belongs to the technical field of basic appliance elements, and aims at solving the problem that the existing tungsten contact is easy to be oxidized to affect the electrical conductivity. In the prior art, the existing tungsten contact is mainly prepared by using pure copper as a solder to perform fusion welding on a rivet type seated nail and a tungsten plate. In this patent, an outer surface of the tungsten contact welding on the seated nail and the tungsten plate is enclosed and connected with a nickel-plate layer as the nickel-plated tungsten contact. The nickel-plated tungsten contact has a simple and practical structure and stable electrical conductivity, is durable in use, and is applicable to cars, motorcycles, electric horn and other electrical appliances. The contact of the patent uses the tungsten plate plus the nickel-plated layer, while the arc ablation resistance of nickel is low, so that the contact is not suitable for a relatively harsh occasion needing a higher working current or voltage. Our test shows that the nickel is served as a switch contact connects or disconnects (switches on or off) with the gold-plated contact. At a room temperature, but when the working current is 300 mA, the switching time is about 4000, then the contact resistance of the switch is significantly increased, or even to completely disconnect the circuit.
U.S. Pat. No. 7,169,215 discloses materials and methods for electroless deposition of copper-molybdenum alloy. An electrical resistivity of the obtained copper-molybdenum alloy containing alkali metal ions and alkaline earth metal ions is lower than 30 microohm per centimeter. The alloy is deposited on a thin copper and cobalt film on a single silicon wafer, a thermal oxide silicon layer, and a silicon substrate. The copper-molybdenum alloy may be used as an interconnecting material on a blocked layer between metal layers and a chip. In these applications, copper-molybdenum alloy may replace copper, but a resistivity of the alloy is higher than that of copper. The present invention discloses the activation of a substrate with a palladium solution, and then copper-molybdenum is subject to chemical deposition on various substrates. The invention does not relate to selective chemical deposition. Due to the worse arc-ablation performance, copper-molybdenum alloy is not an ideal arc-ablation contact material.
U.S. Pat. No. 4,019,910 discloses to prepare an electroless nickel alloy plating bath. The nickel alloys contains boron or phosphorus, and one or more metals selected from tin, tungsten, molybdenum or copper. The chemical plating bath contains an ester complex obtained by reacting inorganic acid with polyhydric acid or alcohol, such as diboron ester, tungstate ester or molybdate ester of glucoheptonic acid. The nickel alloy is mainly constituted by nickel, and the nickel content is generally within the range of about 60% to about 95% by weight. The alloy has excellent mechanical property and corrosion resistance, and some alloys such as phosphorus-containing nickel alloys, in particular nickel-phosphorus-tin-copper alloys, have non-magnetic or non-ferromagnetic property. The polymetallic nickel alloy disclosed by the invention contains a relatively high content of boron or phosphorus. In the case of using as a contact material, the relatively large amount of boron or phosphorus will affect the initial resistance of the contact. Our tests show that, pure nickel, nickel alloy with high content of nickel (such as nickel-copper alloy or monel alloy, nickel-chromium alloy, etc.), nickel-containing stainless steel, or nickel alloy using nickel as the main component by chemical plating, if serving as the contact of the switch, have poor arc resistance and low service life of switch.
US patent application 20090088511 discloses a chemical plating bath used for selectively forming a cobalt-based alloy protective film on an exposed copper wire. The chemical plating bath includes a cobalt ion and another metal ion (tungsten and/or molybdenum), a chelating agent, a reducing agent, a specific surface active agent and a tetramethylammonium hydroxide. The use of the bath disclosed in this invention does not require the use of a copper seed layer (e.g., a palladium layer) prior to chemical plating. The protective film has the ability of anti-diffusion and anti-electromigration. However, such protective film has high cobalt content, and thus is harder and brittle. Due to the arc, the cobalt-based alloy is very easy to produce cobaltous oxide and thus leads to increased surface resistance. The arc ablation resistance of this protective film is not good, so that this protective film is not suitable for manufacturing electrical contacts or contacts.
The invention with U.S. Pat. No. 6,797,312 describes a plating bath containing no alkali metal is used for forming a cobalt-tungsten alloy. The plating bath can be formulated without the use of tetramethylammonium hydroxide. Prior to depositing cobalt-tungsten metal alloy onto the substrate a catalyst such as palladium catalyst is not used for pre-treating the substrate, and the plating bath can be used for obtaining the deposited cobalt-tungsten alloy layer. The cobalt-tungsten alloy contains a lot of cobalt element, not resisting switch arc ablation. The alloy of this invention also does not relate to how to carry out selective chemical deposition.
The invention with an application patent number 201110193369.5 of the inventor provides a “Pitted-surface metal and rubber composite conductive particle” which is formed by adhering a metal surface layer to a rubber matrix or slitting after adhesion. The metal surface layer is a pitted surface and has concave pits or convex points or both the two; the concave pits or convex points are formed on an outer surface, or an inner surface of the metal surface layer, or both the outer surface and the inner surface; the depths of the concave pits are smaller than the thickness of the metal surface layer; and the heights of the convex points are no less than one tenth of the thickness of the metal surface layer. The metal surface layer is made of metal or alloy, the outer surface can be plated with gold, silver, copper or nickel; the rubber base is silicone rubber or polyurethane rubber; a adhesive layer may be between the metal surface layer and the rubber base, and the adhesive layer is a heat curing adhesion agent, a primer or a material the same as the rubber base. Aids such as a coupling agent can be coated on the inner surface of the metal surface layer. The metal surface layer of the invention has high strength and stable conductivity of electricity, the adhesive layer has high strength, and the rubber matrix has sufficient elasticity. The invention does not provide a solution to the problems of arc ablation resistance of the conductive particles. The present invention also does not propose a specific method of obtaining one or more plated layers on the outer surface of the metal surface layer. In the present invention, the pitted skin is plated with precious metals such as gold and silver. Since the surface area is large, the amount of the precious metal is large and the cost is high.